Denitrification is a biofiltration process commonly applied in recirculating aquaculture systems (RAS) for converting nitrate-nitrogen (NO3-N) to nitrogen gas. To meet the heterotrophic conditions required by denitrifing bacteria, fish faeces can be used as an internal carbon source. However, denitrification efficiency depends on the bioavailability of faecal carbon, which varies with faecal composition and thus diet. In this respect, the present study aimed to explore the effect of diet on the denitrification potential of a marine RAS using internal faecal carbon sources.
Four identical RAS (473L) equipped with an up-flow sludge blanket denitrification reactor (DR), were stocked with juvenile European seabass. Fish were fed over a six-week period with two diets in duplo, which consisted of a basal mixture diluted at 15% with a test ingredient: insect meal (IM) and dried distillers grain with solubles (DDGS). Feeding level was gradually increased until week2, when a fixed amount of 50 g feed/RAS/day was established. During the trial, presettled feacal waste produced per system was constantly fed to the respective DR. The total internal carbon supply was estimated by faecal collection via settling in control systems operating without a DR, with the collected faeces being thereafter analyzed for their content in chemical oxygen demand (COD) and total Kjeldahl nitrogen (TKN). The aforementioned control systems were also used to estimate total NO3-N supply to the DRs while nitrogen from branchial and urinary losses (BUN) was estimated based on nitrogen balances calculated for the systems with a DR. Denitrification efficiency was quantified as NO3-N removal based on measurements at the inlet and outlet of the DR over a 24-h period at the end of the trial. Finally, DR sludge was analyzed for its bacterial composition with Illumina MiSeq sequencing targeting the V4 region of the 16S rRNA gene.
Results showed that DRs operated under similar COD:NO3-N ratios regardless of the diet (DDGS: 3.1±0.5; IM: 2.8±0.1, p>0.05) and were equally efficient in removing NO3-N per kg dry matter feed (DDGS: 3.1±0.5; IM: 2.8±0.1, p>0.05). Despite the similar denitrification performance, diet exhibited a tendency to affect bacterial composition (Fig1; p=0.075). A higher abundance of Bacteriodales (p<0.001) and Desulfovibrionales (p<0.05) was observed for IM. The presence of sulphate-reducing bacteria (Sulforovum and Desulfomicrobium) was negatively correlated with the total nitrogen (Total nitrogen=BUN+TKN) supply to the DRs (p<0.05, r=-0.95 and r=-0.96, respectively). Considering that these bacteria produce hydrogen sulphide, which can compromise fish health, it is concluded that faeces originating from IM may be less suitable as an internal carbon for denitrification in RAS.